The present invention relates to nonaqueous electrolyte secondary batteries as generally represented by lithium secondary batteries, and particularly to load characteristic improvements of positive electrode material after cycling.
In recent years, nonaqueous electrolyte batteries which use metallic lithium, alloys capable of storage and release of lithium ions or carbon materials for the negative active material and lithium-transition metal complex oxides for the positive electrode material have been noted as high-density batteries.
The use of a lithium-cobalt complex oxide (LiCoO2), lithium-nickel complex oxide (LiNiO2) or lithium-manganese complex oxide (LiMn2O4), among the lithium-transition metal complex oxides, for the positive active material results in obtaining high discharge voltages of 4 V class, particularly increasing battery energy densities.
Among the above-listed complex oxides useful for the positive active material, a spinel lithium-manganese complex oxide (LiMn2O4) is regarded as promising from viewpoints of price and stable supply of raw material.
However, there still remains a room for improvement in the use of such a lithium-manganese complex oxide (LiMn2O4) for the positive electrode material. Specifically, this spinel complex oxide shows a marked reduction in capacity with charge-discharge cycling, compared to lithium-cobalt and lithium-nickel complex oxides which do not have a spinel structure.
As one solution to this problem, M. Wakihara et al. reports that the reinforcement of a crystal structure by substitution of a dissimilar element, such as Co, Cr or Ni, for a part of Mn atoms in the spinel lithium-manganese complex oxide (LiMn2O4) improves cycle characteristics (see J.Electrochem.Soc., Vol.143, No.1, p.178 (1996)).
However, such substitution has been still insufficient to improve cycle characteristics because of the following reason. As the spinel lithium-manganese complex oxide undergoes expansion and shrinkage during every charge-discharge cycle of a secondary battery, active material particles also undergo expansion and shrinkage. This reduces a strength of the positive electrode and causes insufficient contact of the active material particles with current collector particles, resulting either in the reduced utilization of the positive electrode or in the fall-off of a cathode mix from a current collector, which both have been problems.
Japanese Patent Laying-Open No. Hei 8-45498 proposes a technique for limiting expansion and shrinkage of a cathode mix in its entirety by combining a lithium-manganese complex oxide with a lithium-nickel complex oxide, based on the finding that the lithium-manganese complex oxide undergoes crystal expansion while the lithium-nickel complex oxide undergoes crystal shrinkage when lithium ions are inserted thereinto.
Also, Japanese Patent Laying-Open Nos. Hei 11-3698 and Hei 1-54122 propose a technique for improving electronic conduction of a cathode mix as a whole and thus cycle performance characteristics by combining a lithium-nickel complex oxide, a lithium-cobalt complex oxide and a lithium-manganese complex oxide, based on the finding that the lithium-cobalt complex oxide exhibits a higher electronic conduction than the lithium-manganese complex oxide.
While such combinations achieve improvements to certain degrees, there still remains a room for improving cycle performance characteristics. The inventors of the present application have studied the reduction in capacity with cycling for a positive electrode material (active material) containing a mixture of a spinel lithium-manganese complex oxide and a lithium-nickel complex oxide and found that its load characteristics decrease with increasing cycles. That is, the capacity reduction has been observed to occur when its capacities both initially and after cycles are measured at a relatively high current, e.g., at a 1 C discharge rate, as a result of the reduced load characteristics.
It is an object of the present invention to provide a nonaqueous electrolyte secondary battery which has a high capacity retention and exhibits improved cycle performance characteristics.
A nonaqueous electrolyte secondary battery in accordance with a first aspect of the present invention is characterized as using a mixture of a first oxide and a second oxide for the positive electrode material. The first oxide is a spinel oxide consisting substantially of lithium, manganese, a metal other than manganese, and oxygen. The second oxide is different in composition from the first oxide and consists substantially of lithium, nickel, cobalt, a metal other than nickel and cobalt, and oxygen.
The first aspect of the present invention is described below.
A specific example of the first oxide is an oxide derived via substitution of other element for a part of manganese in a lithium-manganese complex oxide. A specific example of the second oxide is an oxide derived via substitution of cobalt and other element for a part of nickel in a lithium-nickel complex oxide.
The use, in combination, of the first oxide derived via substitution of other element for a part of manganese in the spinel lithium-manganese complex oxide and the second oxide derived via substitution of cobalt and other element for a part of nickel in the lithium-nickel complex oxide is effective to suppress deterioration of load characteristics with cycling. A first reason for this is considered due to the inclusion of dissimilar elements, in theform of a solid solution, that causes active material comprising the first and second oxides to undergo a change in electronic state to the extent that improves electronic conduction of the active material in its entirety. A second reason is considered due to the use, in combination, of the lithium-manganese complex oxide which undergoes crystal expansion when lithium ions are inserted thereinto and the lithium-nickel-cobalt complex oxide which undergoes crystal shrinkage when lithium ions are inserted thereinto, that is effective to maintain stable contact between particles of the first and second oxides during repetitive cycling.
Examples of first oxides include spinel lithium-manganese complex oxides represented by the compositional formula LixMn2xe2x88x92yM1yO4+2 (where M1 is at least one element selected from the group consisting of Al, Co, Ni, Mg and Fe, 0xe2x89xa6xxe2x89xa61.2, 0 less than yxe2x89xa60.1 and xe2x88x920.2xe2x89xa6zxe2x89xa60.2).
Preferably, M1 in the compositional formula LixMn2xe2x88x92yM1yO4+z is at least one of Al and Mg.
Examples of second oxides include complex oxides represented by the compositional formula LiaM2bNicCodO2 (where M2 is at least one element selected from the group consisting of Al, Mn, Mg and Ti, 0 less than a less than 1.3, 0.02xe2x89xa6bxe2x89xa60.3, 0.02xe2x89xa6d/(c+d)xe2x89xa60.9 and b+c+d=1). Preferred among them are those which contain Al in the place of M2 and satisfy 0.1xe2x89xa6d/(c+d)xe2x89xa60.5 in the compositional formula LiaM2bNicCodO2.
The capacity is suitably maintained at high values, if the aforementioned first and second oxides are mixed in the ratio by weight of 20:80-80:20. Within the specified range, the electronic conductivity of the whole is improved and contact between particles of first and second oxides is maintained in a more stable manner, so that deterioration of load characteristics with cycling is suppressed effectively.
The first oxide in the form of a lithium-manganese complex oxide preferably has a mean particle diameter of 5-30 xcexcm. The second oxide in the form of a lithium-nickel-cobalt complex oxide preferably has a mean particle diameter of 3-15 xcexcm. The combination thereof is most preferred. Preferably, the first oxide has a larger mean particle diameter than the second oxide. If the mean particle diameter of each oxide is maintained within the above-specified range, contact between particles of those complex oxides is maintained at a higher degree of occurrence to thereby improve the electronic conduction of the mix in its entirety. Also, expansion and shrinkage are balanced more effectively between those complex oxides so that contact between particles of those complex oxides is maintained in a more stable manner. As a result, the deterioration of load characteristics with cycling can be suppressed. The mean particle diameter is determined by observing the positive active material or cathode mix with a scanning electron microscope (SEM), measuring londitudinal dimensions, of 5 particles among active material particles present in a 100 xcexcm square and calculating a mean value which is taken as a mean dimension for all particles.
If the above-described configurations and constructions are satisfied properly, nonaqueous electrolyte secondary batteries can be provided which are highly reliable and show little deterioration of load characteristics with charge-discharge cycling.
A nonaqueous electrolyte secondary battery in accordance with a second aspect of the present invention is characterized as using a mixture of a first oxide, second oxide and third oxide for the positive electrode material. The first oxide is a spinel oxide consisting substantially of lithium, manganese, a metal other than manganese, and oxygen. The second oxide is diffent in composition from the first oxide and consists substantially of lithium, nickel, cobalt, a metal other than nickel and cobalt, and oxygen. The third oxide is diffent in composition from the first and second oxides and consists substantially of lithium, cobalt, a metal other than cobalt, and oxygen.
The second aspect of the present invention is described below.
The first and second oxides used in the first aspect of the present invention are applicable to this second aspect.
Specific examples of third oxides include lithium-cobalt complex oxides and oxides derived via substitution of other element for a part of cobalt in lithium-cobalt complex oxides.
Also in the second aspect, the use of the same first and second oxides used in the first aspect suppresses the deterioration of load characteristics with cycling for the same reasons adduced in the first aspect.
In the second invention, the third oxide is further mixed with the first and second oxides. The increased electronic conductivity of the third oxide relative to the first and second oxides is effective to further suppress deterioration of load characteristics with cycling (See, for example, M. Menetrier et al., xe2x80x9cThe Second Japan-France Joint Seminar on Lithium Batteries, Nov. 23-24, 1998, Morioka, Japanxe2x80x9d, p.83).
Examples of third oxides include complex oxides represented by the compositional formula LieM3fCo1xe2x88x92fO2 (where, M3 is at least one element selected from the group consisting of Al, Mn, Mg and Ti, 0 less than e less than 1.3 and 0xe2x89xa6f less than 0.4). Preferred among them are those which contain at least one of Mg and Ti in the place of M3 and satisfy 0.02xe2x89xa6fxe2x89xa60.2 in the compositional formula LieM3fCo1xe2x88x92fO2.
The capacity is suitably maintained at high values when the aforementioned first, second and third oxides are mixed in the weight ratio of (first oxide):(second oxide+third oxide)=20:80-80:20. Within the specified range, the electronic conductivity of the whole is improved and contact between particles of the first, second and third oxides is maintained in a more stable manner, so that deterioration of load characteristics with cycling can be suppressed.
The capacity is optimally maintained at high values when the aforementioned second and third oxides are mixed in the weight ratio of (second oxide):(third oxide)=90:10-10:90. Within the specified range, electronic conductivity of the whole is further improved and deterioration of load characteristics with cycling is further suppressed.
The first oxide in the form of a lithium-manganese complex oxide preferably has a mean particle diameter of 5-30 xcexcm. The second oxide in the form of a lithium-nickel-cobalt complex oxide preferably has a mean particle diameter of 3-15 xcexcm. The third oxide in the form of a lithium-cobalt complex oxide preferably has a mean particle diameter of 3-15 xcexcm. The combination thereof is most preferred. Preferably, the first oxide has a larger mean particle diameter than the second and third oxides. If the mean particle diameter of each oxide is maintained within the above-specified range, contact between particles of those complex oxides is maintained at a higher degree of occurrence to thereby improve the electronic conductivity of the mix in its entirety. Also, expansion and shrinkage are balanced more effectively between those complex oxides so that contact between particles of those complex oxides is maintained in a more stable manner. As a result, the load characteristic deterioration with cycling can be suppressed. The mean particle diameter can be determined in the same manner as described in the first aspect.
If the above-described configurations and constructions are satisfied properly, nonaqueous electrolyte secondary batteries can be provided which are highly reliable and show little deterioration of load characteristics with charge-discharge cycling.
The matters in common with the first and second aspects of the present invention are below described as xe2x80x9cpresent inventionxe2x80x9d, collectively.
The battery materials other than the positive electrode material, for use in the present invention, can be selected from those known in the prior art as useful for nonaqueous electrolyte secondary batteries, without particular limitations.
Examples of negative electrode materials include lithium alloys such as metallic lithium, lithium-aluminum alloys capable of storage and release of lithium, lithium-lead alloys and lithium-tin alloys; carbon materials such as graphite, coke and calcined organics; and metal oxides having potentials more negative than the positive active material, such as SnO2, SnO, TiO2 and Nb2O3.
Examples of nonaqueous electrolyte solvents include high-boiling solvents such as ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC) and butylene carbonate (BC); and mixed solvents thereof in combination with low-boiling solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), 1,2-diethoxyethane (DEE), 1,2-dimethoxy ethane (DME) and ethoxymethoxyethane (EME).